2005 Ford Explorer takes a long time to start?

I have a 2005 Ford Explorer with around 40,000 miles and lately when I go to start it, the engine will rev for longer than usual and then it ll turn on. When I bought it used it started up quick like it s supposed to but I m worried if this is serious or not? It starts everytime, it just takes a while, which is why I haven t taken it in yet. Help!

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